Right before the final fight, we see Loki and Stark discussing the terms of the upcoming battle, bantering back and forth between who will win.

It is then that Loki gets the idea of turning Stark against his comrades like he did with Hawkeye, as well as a bunch of other S.H.I.E.L.D. agents at the beginning of the movie.

But, it doesn't work at all.

Is this because of Stark's Arc Reactor countering the Cosmic powers somehow, because Stark is incorruptible (which doesn't make much sense), or possibly because simply he was touching the Arc Reactor and not Stark's flesh?

3 Answers
3

When Loki tries to take control of Stark he actually hits the ARC reactor and not the flesh over his heart. Your graphic explains it best with the "Tink, Tink" sound showing that Loki is actually hitting the metal of the reactor.

In the Avengers director commentary, Joss Whedon states it fails to work because the ARC reactor prevents the staff from reaching Tony's heart.

When he takes over Hawkeye at the beginning of the film, Loki says Barton "has heart". Since Stark technically doesn't have a heart, at least not in the way ancient Asgardian magic would understand, Loki's spell doesn't work.
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jeffmJun 13 '12 at 18:30

Couldn't he just connect above his arc reactor or below. Oh may be the idea didn't come to his mind :)
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AnshuSep 22 '12 at 17:44

There is a reference somewhere in the film that the cube creates a doorway using energy that is similar to the ark reactor used by Tony. This is why Loki puts the doorway device on top of Tony's building, because it's powered by it's own reactor. There is another reference somewhere that the energy in the staff is the same as the cube. I took all this to mean that the reactor on Tony's chest was somehow interfering.
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ThinkingMediaSep 26 '12 at 20:22

3

Avengers is now out on home media. The director commentary actually explicitly does confirm this.
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user209Sep 27 '12 at 1:20

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@jeffm I know this was a long time ago, but not at any point do the films ever say tony doesn't have a heart, nor is the arc a replacement of his heart, it is there just to keep shrapnel from penetrating it
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FlauntingSep 2 '14 at 8:33

Remember: Stark's father was studying the power of the cube. In Iron Man 2 he said only Tony could accomplish the creation of this new particle. It could have been a tesseract particle of some sort that Tony made with his new reactor. This could be why the spear of Loki didn't affect Stark. Just the same as the spear could affect the closing of the portal.

While this answer seems to be pure speculation, I really like the idea of the Tesseract being the origin of Tony's new particle.
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user209Sep 1 '12 at 17:15

Speculation ...maybe but in the comics the teseract is a tool of the beyonders to study other races potential...it is also a power from a domain outside our existence ..as Jarvis said a particle Impossible to create..given Jarvis a self aware AI has knowlege of nearly all known elements in our universe ...this speculation makes the most sence...his reactor also runs out like a hydra weapon
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user2000Sep 2 '12 at 0:09

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Howard stark Captian America when studying the hydra crystal particle befor it exploded said...alpha beta ray neutral....boom!..write that down...he initially had a small piece of the puzzle in a short period of time ...now tink about decades with it...makes more sence that he studied it enough to figure he couldn't create the particle with that days current technology...so it was tasked to tony
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user2000Sep 2 '12 at 0:23

Even though the Spear and Cube are known as mystical and magical items, they are used to tap into a source of almost universal convertable energy. An energy similar to the emitted power generated by the Arc Reactor. The two energies, if not even the same seem to interfere each other.

It is known that the Arc Reactor is able to store and absorb large amounts of energy. So it is possible that it absorbed the low frequency energy normally used to correspond and control the victims brain patterns.

As seen the Arc Reactor is not only touched by the spear/scepter but it's attracting it in a magnet-like manner. In my opinion the reactor is emitting an electromagnetic field that is attracting the spear as well as interfering the energy projection which is causing the spear's ability to control the victim's brain patterns.